1. Field of the Invention
This invention relates to new polyurethane stoving lacquers which are dissolved or dispersed in a predominantly aqueous medium, and to the use thereof for the production of lacquer films and coatings.
2. Description of the Prior Art
Those lacquers wherein the solvent is based partly or completely on water are becoming increasingly important in technology for reasons of pollution control and for saving organic solvents which are dependent on petroleum. Attempts have been made for a long time to produce aqueous lacquers which are to be equal in their properties to lacquers dissolved in organic solvents. Up until now, it has been impossible to fully achieve this objective. For this reason, aqueous lacquers, apart from cationic electrophoretic enamels, are only used to a limited extent for the production of high-grade lacquers.
One of the main problems which arise when aqueous lacquer systems are used is the poor pigmenting ability thereof. In the case of pigment contents which are required to achieve a good covering power, water lacquers usually lose their gloss. The other properties in terms of lacquering, for example elongation, impact elasticity, adhesion and corrosion prevention, also frequently deteriorate as the pigmenting increases.
In order to overcome these problems of pigmenting, it is proposed in DE-AS No. 2,507,884 to dissolve polyacrylate or polyester lacquer binders which contain carboxyl groups in low-boiling organic solvents before they are dispersed in water, and to work the pigments into this solution. The organic pigment dispersion which is produced in this manner is dispersed in water once the carboxyl groups have been neutralized, and the organic solvent is removed by azeotropic distillation. This method suffers from various disadvantages. First of all, the detour via the organic pigment dispersion with subsequent azeotropic distillation represents a considerably increased technical expense compared to the conventional operation for lacquers. Secondly, the pigmenting restricts the manufacturer's ability to alter the formulation of the lacquers to the time before the lacquer is converted into an aqueous dispersion, because once the mixture had been dispersed, the pigmenting thereof cannot be changed.
Thus, an object of the present invention is to provide an aqueous lacquer system which does not suffer from the above-mentioned pigmenting problems. In the aqueous phase, it should be able to be mixed with the quantities of pigments which are required to achieve an effective covering power according to the methods conventional in lacquer technology and, after being applied in a conventional manner, it should produce lacquers which have a good gloss and which are distinguished by good elongation, good adhesion and an effective corrosion prevention with a high hardness level.
This object may be achieved by providing the lacquer systems according to the present invention which are described in more detail in the following. This invention is based on the surprising observation that aqueous solutions or dispersions of heat-hardenable mixtures of polyurethane prepolymers containing carboxylate groups and blocked isocyanate groups and polyurethane prepolymers containing carboxylate groups and hydroxyl groups are not affected by the pigmenting problems described above if these polyurethane prepolymer mixtures contain specific polyether segments which are defined in more detail in the following, in addition to specific polyester segments which are also defined in more detail in the following.
The fact that the good pigmenting ability of the products according to the present invention is actually unusual for aqueous lacquer systems is also revealed by comparative experiments with conventional polyurethane dispersions, for example with the thermally cross-linkable water lacquers which may be obtained according to EP-OS No. 75 775 and which contain polyurethane-polyureas having carboxylate groups and hydrophobic blocked polyisocyanates.
Combinations of water-soluble or water-dispersible blocked polyisocyanates which contain carboxylate groups and water-soluble or water-dispersible organic compounds which have isocyanate-reactive hydrogen atoms are known in principle and are described in DE-PS No. 2,456,469. However, this patent does not disclose the measures which are essential to the present invention, which require combining selected water-soluble or water-dispersible NCO-prepolomers having blocked isocyanate groups with specific water-soluble or water-dispersible polyurethane prepolymers having alcoholic hydroxyl groups, whereby specific polyether or polyester segments must be present in a chemically incorporated form in the above-mentioned components.
Furthermore, DE-OS No. 2,642,073 describes specific hydrophilic polyurethane prepolymers which may contain carboxylate groups and blocked isocyanate groups, inter alia, and are dispersed in water as solids. The coatings produced therefrom are self-cross-linking, i.e., they are not used in a combination with reaction components.
The two above-mentioned patents do not discuss the pigmenting of the coating agents. Therefore, they naturally do not contain any references to measures for alleviating pigmenting problems.
A few later publications also relate to polyurethane prepolymers which contain carboxylate groups and blocked isocyanate groups, and to the use thereof for the production of aqueous lacquers (DE-OS No. 2,708,611, EP-PS No. 22 452 and U.S. Pat. No. 4,403,085). However, the measures which are essential to the present invention to achieve a good pigmenting ability are not described in these publications, especially since no disclosure is directed to the pigmenting of the aqueous lacquer systems presented therein.
Finally, DE-OS No. 3,234,590 describes water-soluble urethane prepolymers having blocked isocyanate groups and containing ionic groups, for example carboxylate groups, which are suitable as adhesives and impregnating agents. A combination of these blocked isocyanates with reaction components of the type to be used according to the present invention for the production of lacquers is not described. Consequently, that patent application is also not directed to the pigmentation of lacquers.